The present invention relates generally to cameras and, more particularly, is directed to an automatic focusing device for a camera.
An automatic focusing device for use in a camera according to the prior art includes a camera lens focusing mechanism located on the side of a shutter unit of the camera. Specifically, a drive ring is rotatably mounted to a base plate disposed on the side of the shutter unit, and is driven by a stepper motor through an idle gear. According to a preset program, the stepper motor is rotated forwardly or rearwardly for focusing, exposure, or return. In response to the rotation of the stepper motor, the drive ring is rotated forwardly or rearwardly. A cam, which acts on a sector opening lever at a point outside the focusing region to open or close sectors, protrudes from the periphery of the drive ring.
A range ring drives the camera lens via a lens driving portion thereof to position the lens at the correct focal point. The range ring is rotatably mounted to the base plate in the same way as the drive ring. The range ring has a sector gear in mesh with a pinion that is integral with a ratchet wheel. Further, the range ring is biased in a clockwise direction by a range spring so as to rotate with the drive ring.
A locking lever is biased by a lever spring and is pivoted to a pin on the base plate, with an iron member coupled to one end of the lever. The lever is attracted and held to an electromagnet via the iron member. Thus, when the electromagnet is deenergized, the locking lever is rotated clockwise, and a pawl at the front end of the locking lever enters between the teeth of the ratchet wheel. As a result, the range ring is locked at the focal point.
In operation, the stepper motor is first rotated rearwardly, so that the ratchet wheel is rotated clockwise via the drive ring and the range ring, and the locking lever is pushed upward. At this time, the electromagnet is energized to hold the locking lever at a position where the teeth of the ratchet wheel are not engaged.
Then, the stepper motor is rotated forwardly to perform a focusing operation, and this rotation is transmitted to the driving ring. The range spring rotates the range ring clockwise so that the range ring follows the drive ring in rotation. The rotation of the range ring causes a scanning lever to scan a light-emitting device held by it. A light-receiving device receives the light from the light-emitting device as the light reflected from the subject. When the lens is brought into focus, the light-receiving device delivers a focus signal and provides a display of information about the distance in the viewfinder. The focus signal deenergizes the electromagnet and rotates the locking lever to the right so as to engage the ratchet wheel, to lock the range ring. Thus, the camera lens is placed in the focal position.
If the stepper motor rotates further, the drive ring turns clockwise, causing the cam portion to rotate the pin of the sector opening lever to the right. Before the pin rotates back to the left, the range ring which is controlled by the stepper motor is disconnected from the stepper motor. Then, the drive ring opens or closes the shutter in a given manner, as determined by the stepper motor, that is, the stepper motor is rotated either forwardly or rearwardly in response to phase pulses produced by a stepper motor driving circuit.
However, the aforementioned prior art automatic focusing device has no means of checking if the electric signal, or phase pulses, supplied to the stepper motor from the stepper motor driving circuit is correctly transformed into mechanical movements, that is, correct forward or rearward rotation of the motor. For example, if movement of the camera lens focusing mechanism which is governed by forward or rearward rotation of the motor is impeded by a person, the circuit controlling the stepper motor driving circuit regards the operation of the lens focusing mechanism as correct. Then, the control circuit enables a sequence of further operations including control of the shutter mechanism. As a result, the photograph is out of focus or other problems result.